Densitometers are well known devices which are employed to scan a sample and provide an output signal or graphical display indicative of the optical density, transmittancy, absorption or the like of the scanned sample. One well known use of the densitometer involves scanning a sample of blood which has been prepared by the electrophoresis process. Electrophoresis of blood samples isolates the various proteins in the blood, known as albumin, alpha-one globulin, alpha-two globulin, beta-globulin and gamma-globulin. The electrophoresis technique separates these proteins from each other, following which the sample is scanned by an optical density pick-up. Each of the proteins exhibits a different light absorption characteristic or pattern and the patterns are graphically displayed by the densitometer to indicate the presence and quantity of each of these proteins.
The electrical analog signals generated by the optical pick-up, when graphically displayed, exhibit a series of peaks and valleys. In the analysis of blood, the area under the optical density curve and bounded by two adjacent valleys separated by one peak, is representative of the quantity of each protein in the sample and is referred to as the sample fraction. Of primary importance is the relative percentage of each protein and the selection of these fraction boundaries, i.e., the precise location of these valleys is somewhat arbitrary and results in inaccurate analysis of the blood sample. This problem is not unique to evaluation of blood samples but is common to optical and magnetic density evaluations, and, in fact, to all evaluations of analog data.
The electrophoretic process involves applying a plurality of blood samples or other substance which is to be electrophoretically separated into constituent components, on a plate. Normally a plurality of samples, typically from different patients, are applied to a single plate using an applicator or a template. The samples are typically arranged in a column and each sample migrates into the constituent components, e.g., proteins, to form rows of optical density patterns. The constituent components of the various samples migrate substantially the same distances, depending upon the thickness of the plate, the strength of the buffer employed, the length of time of process and the voltage applied during the electrophoresis process. Consequently, the end result is a plate having a series of rows of optical density patterns corresponding to each sample (patient) with like constituent components forming columns.
The plates are scanned in a densitometer system by arranging the plates in rows and columns on a flat carrier which is in turn placed on a moveable carriage beneath the optical pick-up. Scanning is performed by moving the carriage in mutually orthogonal directions in a manner such that the columns of consitiuent components are sequentially scanned in order to generate the optical density data which is used to produce an analog graphical display in the form of a curve, as previously discussed.
In the past, it has been necessary to carefully align the plates on a carrier with reference to preselected locations so that the preprogrammed scanning path passed through the centers in each optical density component of the columns. In some cases, results were less than completely satisfactory for several reasons. In some cases, the operator may skew the position of one plate relative to the other thus offsetting the columns of pattern components. In other cases, offset may occur because the degree of pattern migration between different plates may vary as a result of differences in the thickness of the plate, the strength of the buffer used, the length of time of process and the voltage applied during electrophoresis of the sample. Consequently, not all of the pattern components are scanned along their centerlines; this may result in cross talk between adjacent pattern components and in some cases, some components may not be picked up.
In addition to the foregoing problems, previous procedures for determining the precise location of the pattern components, i.e., fraction boundary location and for determining the overall length of each pattern were also subject to improvement. In the past, fraction location was performed by digitally comparing sample amplitudes to determine when the slope changed along the scanning path. Moreover, the previous scanning procedure sometimes resulted in overscan of the pattern length, thereby increasing overall scanning time.